Real Tank engines
Tank Engines, carrying their own fuel and water without tenders were used in the 1800s mostly for moving cars around terminal stations or for assisting very heavy trains in tough pulls.
The big engines on the Island of Sodor pull coal cars, like real steam engines do, but Thomas has square tanks beside his cylindrical boiler and carryies his own coal in a small bin behind the cab. Many people think of the luxury and romance of rail travel and think of the beautiful dining cars, and sleeping cars. Truthfully the railway system was and is about much more than luxury travel. With the rise of the highway system and peoples ability to afford and refinance car purchases travel by rail has declined, however, the rail system is still an important part of the economy. Trains drive industry and coal cars, shipping cars, and tank engines like Thomas are an important part of the rail system today. The square tanks beside the boiler are what make Thomas a "tank engine." In real life, the tanks might hold between 1,000 and 2,000 gallons of water. A real tender, or coal car carries both coal and many thousands of gallons of water. This water is needed because most steam engines vent their used steam through the smoke stack rather than condensing and reusing it. All engines except tank engines need to carry these coal/water cars right behind the engine.
Tank engines came about as a way to handle short lines and switching duties in a train yard. The engine carried a small amount of coal behind the cab and water in its tanks. A tank engine is therefore self-contained and does not need the coal/water car. This makes it lighter, smaller and less expensive, but gives it a limited range before needing to be re-coaled and re-watered. Tank engines were not very common in the United States, but were very common in England.
Percy is another story and when he was originally drawn by someone other than Rev. W. Awdry, he was illustrated, Awdry claimed Percy looked like "a green caterpillar with red stripes". This infuriated C. Reginald Dalby, the illustrator of The Railway Series, who then resigned. I say all this to say that Percy is somewhat muddled... Not a tank engine but really to small to be an industrial locomotive. (The television series, however, utilized the original design.)